Ventilation: what size of fan you need

One of the most common questions i see new growers posting....
What size of extraction fan and carbon filter do i need for my tent?
Extraction fans are rated in m3/hour

Heres the simple answer for some of the most common tent sizes.

0.6m x 1.2m x 2m = 115m3/hour
1m x 1m x 2m =160m3/hour
1.2m x 1.2m x 2m = 230m3/hour
1.5m x 1.5m x 2m = 360m3/hour
1.2m x 2.4m x 2m = 460m3/hour
2m x 2m x 2m = 640m3/hour
2.4m x 2.4m x 2m = 920m3/hour

So find a fan that meets the airflow needed for your area if it takes a 125mm/5inch fan then you need the same size of carbon filter and ducting.

If you want to learn how to work this out for yourself in any size of tent or grow room keep reading.

Firstly work out the volume of your grow space (length x width x height) then multiply this number by 60. This gives you the maximum amount of air you need to exchange each hour.

So for example a 1.2 x 1.2 x 2m tent gives you a volume of 2.88m3, multiply this by 60 and you have 172.8m3/hour.
At the moment, a 100mm (4”) RVK100A1 fan seems suitable for this scenario with an air moving capacity of 175m3/hour. However, it does not consider the impact of attaching a carbon filter and ducting…

To factor in the expected fan speed reduction of around 25%, multiply the original calculation by 1.33.

So 172.8m3/hour x 1.33 = 230m3/hour.

Other factors to consider.

- Add 20% for a warm attic

- Minus 15% for a cool basement

- South facing rooms will tend to be hotter than north facing rooms – increase the size of fan and use a fan controller for exposed areas

- Add 20% for long ducting runs and/or multiple bends

- If incoming air is 25oC or above then the room will become hot irrespective of the ventilation – an OptiClimate System or Air Conditioning Unit is the only solution

- Minus 25-30% for air cooled lighting

 

[Blitzen]

Hi all! long time lurker first time poster

I'm a novice grower but I design odor control systems for many industries like plastic extrusion vents where the odors and VOC's are pretty high. Working out your carbon filtration is really just a matter of doing some math. Hopefully some of you will find this useful.

For carbon to work properly the smelly air needs a certain amount of "dwell time" in the carbon. I see a lot of people use fan systems that are too poweful and the air gets pulled too quickly through the carbon. Get in touch with these guys http://www.generalcarbon.com/ and ask for a carbon pellet that is formulated for cannabis. potassium permanganate and coconut Shell are another odor absorption medias that work well. There are literally 100's of different carbon formulas for VOC and odor control.

To work out your cfm needed for 10 air changes per hour you multiply your fan systems cfm x 60 and divide by the volume of the space. For example...if a 4" cent' fan has a cfm rating of 187cfm - if your grow space is 6' x 8' x 8' then 187x60=11,200/384 = 29.1 ach.

So..a 4" fan that does 187 cfm running on full speed with an line carbon filter is pulling too fast and the air is not getting adequate "dwell time"...but that same fan running at 1/2 or 1/3 speed is going to be much more effective.

I hope this info is useful..

This is the stuff

 

[Blitzen]

Awesome information, this is the stuff that saves lives!!!! THANKYOU!!!

 

[Simplicio]

Hi all! long time lurker first time poster

I'm a novice grower but I design odor control systems for many industries like plastic extrusion vents where the odors and VOC's are pretty high. Working out your carbon filtration is really just a matter of doing some math. Hopefully some of you will find this useful.

For carbon to work properly the smelly air needs a certain amount of "dwell time" in the carbon. I see a lot of people use fan systems that are too poweful and the air gets pulled too quickly through the carbon. Get in touch with these guys http://www.generalcarbon.com/ and ask for a carbon pellet that is formulated for cannabis. potassium permanganate and coconut Shell are another odor absorption medias that work well. There are literally 100's of different carbon formulas for VOC and odor control.

To work out your cfm needed for 10 air changes per hour you multiply your fan systems cfm x 60 and divide by the volume of the space. For example...if a 4" cent' fan has a cfm rating of 187cfm - if your grow space is 6' x 8' x 8' then 187x60=11,200/384 = 29.1 ach.

So..a 4" fan that does 187 cfm running on full speed with an line carbon filter is pulling too fast and the air is not getting adequate "dwell time"...but that same fan running at 1/2 or 1/3 speed is going to be much more effective.

I hope this info is useful..

Very useful info.
The accepted range of values for number of air exchanges per hour is probably too high, and your 10 exchanges per hour sounds more realistic.